How to Open Locked Blue Chests
What are Blue Chests?
Blue chests are special types of chests that contain rare items, weapons, armor, and even songbooks. You will not be able to open blue chests until a bit later in the game once you start upgrading Evermore Castle.
Opening Blue Chests
Acquire the Spring Lock Spell from Evermore Spellworks (Chapter 5)
To open blue chests, you must clear Chapter 4 of the main story to be able to build and upgrade Evermore Castle. Afterwards, you must upgrade your kingdom to level 2 by recruiting at least 15 citizens and building more structures, including the Evermore Spellworks. From here, you must upgrade the Evermore Spellworks to level 3 which allows you to acquire the Specific Spell Dev: Spring Lock.
Upon getting the Spring Lock spell, you will then be able to unlock blue chests found throughout the world map. Note that blue chests in the Dreamer Door mazes cannot be opened this way as they require purple fragments (dropped by enemies in the Dreamer Door mazes) to unlock them.